A magnetoresistive element having a ferromagnetic tunnel junction is also called a magnetic tunnel junction (MTJ) element, and a write method (spin transfer torque writing method) using the spin-momentum-transfer (SMT) has been proposed as a write method of the MTJ element.
There is a method of using a so-called perpendicular magnetization film having the axis of easy magnetization in a direction perpendicular to the film plane as a ferromagnetic material forming the magnetoresistive element. When using magnetocrystalline anisotropy in a perpendicular magnetization arrangement, the element size can be made smaller than that of an in-plane magnetization structure because no shape magnetic anisotropy is used. In addition, dispersion in the direction of easy magnetization can be decreased. By using a material having large perpendicular magnetocrystalline anisotropy, therefore, it is expected to realize both smaller element size and lower switching current while maintaining the thermal stability.
The perpendicular magnetization arrangement has the problem that a large strayed magnetic field is generated as small size progresses. A strayed magnetic field from a fixed layer (reference layer) shifts a switching field in a storage layer in a direction in which a parallel arrangement is stable, thereby increasing a switching current. To avoid this problem, a magnetic layer (shift cancelling layer) having a magnetization direction antiparallel to that in the fixed layer must be added to cancel the strayed magnetic field from the fixed layer.
If a large strayed magnetic field is generated as small size progresses, it becomes difficult for the shift cancelling layer to adjust the strayed magnetic field from the fixed layer. Accordingly, it is necessary to perform adjustment so as to decrease the saturation magnetization and film thickness of the fixed layer, and adjust the structure so as to increase the saturation magnetization of the shift cancelling layer.